1. Field of the Invention
The present invention relates to an electric double layer capacitor, particularly an electrode therefor.
2. Description of the Background
An electric double layer capacitor, especially a coin-shaped capacitor, employing an electric double layer formed at the interface between a polarizable electrode formed at the interface between a polarizable electrode and an electrolyte, has found a rapid increase in the demand as a memory backup power source in recent years. On the other hand, also in the field of application where a large capacity is required, such as, a power source for electric automobiles, it is desired to develop an electric double layer capacitor having a large capacity per unit volume, a low internal resistance, a high energy density and a high output density. Reduction of the internal resistance is desired also for the electric double layer capacitor to be used as a memory backup power source.
An electrode for an electric double layer capacitor is prepared e.g. by kneading an activated carbon powder into a slurry by means of a solvent for an electrolyte such as sulfuric acid, followed by press molding (U.S. Pat. No. 3,288,641). However, the electrode obtained by this method has a rigid porous structure and is susceptible to cracking or fracture and thus is not durable for use in a long period of time. Whereas, a carbon paste electrode has been proposed which is made of a viscous material having a binder of polytetrafluoroethylene optionally added to a mixture comprising an activated carbon powder and an electrolyte (JP-B-53-7025, JP-B-55-41015). This electrode is flexible and scarcely susceptible to cracking or fracture, but the shape-maintaining property is inadequate, and a cell having a special structure is required to supplement the strength for practical use.
Further, in order to obtain an electrode which is scarcely susceptible to cracking or fracture and excellent in the shape-maintaining property, a method has been proposed wherein a kneaded product comprising a carbonaceous material, a binder such as PTFE and a liquid lubricant, is preliminarily molded and then stretched or rolled to obtain an electrode molded in a sheet form (JP-A-63-107011, JP-A-2-235320). However, by this method, an electrode sheet having an adequately high strength even if the thickness is thin, can not be obtained, and there has been a problem from the viewpoint of the productivity or durability, when it is applied to an electrode for an electric double layer capacitor especially for a large capacity application.
The present invention has been made to solve the above mentioned conventional problems, and it is an object of the present invention to provide an electrode which has a high strength even in a sheet form having a thin thickness and which is useful for an electric double layer capacitor having a high capacitance density and a low internal resistance, particularly an electrode suitable for an electric double layer capacitor for a power application.
The present invention provides an electrode for an electric double layer capacitor, which is a sheet electrode comprising a carbonaceous material and a polytetrafluoroethylene and whereby K calculated by the following formula 1 from X-ray diffraction peak intensities measured by CuKxcex1-ray is at most 33:
K=IPTFE/(ICARBONxc3x97(CPTFE/CCARBON))xe2x80x83xe2x80x83(1)
where IPTFE is the X-ray diffraction peak intensity of the polytetrafluoroethylene in the vicinity of 2xcex8=18xc2x0, ICARBON is the X-ray diffraction peak intensity of the carbonaceous material in the vicinity of 2xcex8=43xc2x0, CPTFE is the concentration by weight of the polytetrafluoroethylene in the electrode, and CCARBON is the concentration by weight of the carbonaceous material in the electrode.